1. Field of the Invention
The invention relates to a hydraulic control in a hydraulic system for the operation of a machine tool such as a press for processing material of any desired type, such as presses and/or shears, in particular for shears for cutting metal scrap, such as “scrap shears”.
2. Prior Art
Machine tools such as presses essentially comprise                a column having at least one hydraulic cylinder guided and fastened therein for driving a tamper, pressing or holding down the material, toward a table and at least one hydraulic cylinder guided and fastened therein for driving a tool, finally processing and cutting the material, toward a fixed tool in said table, and        a “hydraulic control” with tank, pumps, control blocks, valve elements and nozzles and also the hydraulic medium for the cyclic operation of the first and second cylinders.        a “hydraulic control” with tank, pumps, control blocks, valve elements and nozzles and also the hydraulic medium for the cyclic operation of the first and second cylinders.        
It is known to provide means in the hydraulic control which generate a damping pressure for damping a “(cutting) impact” at least in one of the hydraulic cylinders at the end of its working stroke, such as, for example, after the severing of metal scrap.
The general problematic nature of these (cutting) impacts has been known for a long time and has already been described in detail in publications DE 2808091A1, DE 2824176A1, DE 2909119A1, DE 3534467A1, DE 2221290C3, DE 2928777C2, DE 3112393C2 and DE 19529134A1.
A useful and already known solution to the problem can be gathered from the preamble of the main claim of EP 0765203B1 (also published as U.S. Pat. No. 5,832,807), in which case this embodiment is to be advantageously augmented by the characterizing features such as                the control pressure corresponding to the damping pressure can be built up by inflow of control oil from the piston-side cylinder space of the working cylinder via a bypass line into the control space, and        a check valve which shuts off flow toward the piston-side cylinder space is arranged in the bypass line.        
The previously known hydraulic control can be effectively presented in the following steps with regard to the outflow relevant to the damping of the (cutting) impact (with reference to EP 0765203, also published as U.S. Pat. No. 5,832,807):                1. The cylinder descends with the cutter. A counter-pressure is generated in the rod space 12 of the cylinder by means of a pressure valve 54. The valve 54 interacts with a valve 31 as a two-stage pressure-limiting valve, the valve 31 constituting the main stage. A valve 38 is held by a spring in the position P-A, since the control pressure in line 40 at this instant is still low. The counter-pressure in the rod space during the downward travel is necessary in order to hold the load attached to the cylinder. Without this counter-pressure, the cylinder, due to the weight of the piston rod and the attached load G, would travel faster than predetermined by the pump quantity due to gravitational effects. This would result in a vacuum in the piston space 10, and this vacuum can cause problems with the seals of the cylinder.        2. The cylinder, or the cutter, comes down on the material. The pressure in the piston space 10 increases, since at this point the cylinder is no longer moved. The pump continues to deliver oil, which leads to a pressure increase in the piston space 10. This pressure increase continues via the control line 62, the valve 58 and the line 40 and leads to shifting of the valve 38 into position A-T. Accordingly, the control space of the valve 31 is connected via a nozzle 21 and the valve 38 to the tank; and the valve 31 opens against the spring 66. As a result, the previously required counter-pressure in the annular space 12 of the cylinder collapses. The cylinder can now apply the full desired force, defined by pressure times piston area, for cutting the material. The cutter cuts the material when the cutting force is reached.        3. The cutter is accelerated by the oil volume, acting as a spring, in the piston space. Immediately after the cutting, the pressure in the piston spaces 10 collapses through expansion. At the same time, the valve 38, in the absence of control pressure, shifts back into basic position P-A. The oil flow produced by the accelerated cylinder is partly directed via the nozzle 36 and the valve 38 to the control space of the valve 31. The top pressure on the annular-space side is limited, as in section 3.1, via the two-stage pressure-limiting valve (54/31). This counter-pressure already constitutes a (cutting) impact damping.        4. As can be seen under 2, before cutting, the pressure in the piston space 10 is high and the pressure in the annular space 12 is relieved toward the tank. Since the pressure in the piston space 10 is now higher than in the annular space 12, oil flows via the line 68 and the nozzles 70 and 36 and the check valve 72 from the piston space to the annular space and thus via the valve 31 to the tank. This oil flow multiplied by the applied pressure can adversely produce an unintentional power loss.         During the cutting (as described in 3), the pressure in the piston space 10 collapses, at least partly. Since the pressure in the piston space 10 has decreased and pressure has been generated in the annular space by the accelerated cylinder, there is the risk that no oil can flow via the line 68. The pressure gradient then becomes opposed to the check valve.         The line 68 will therefore also lead to a mutual effect instead of assisting damping. Furthermore, it is to be emphasized that the valve 38 must first of all shift into the basic position for the desired (cutting) impact damping to come into effect. The precondition for this is that the pressure in the line 40 (thus previously in the piston space 10) must have collapsed. Thus, in particular cases, no oil can flow via the line 68 and accordingly via the valve 38, since this at best requires additional means on account of the circuit logic and the oil required is fed via the nozzle 36 of the pilot control.        
In addition, the (cutting) impact damping present per se is accordingly to be assisted with simple means by the 4th step, which, however, does not appear possible without further means or may lead to further complications.
The hydraulic controls already realized in industrial practice with the features (in accordance with the steps 1 to 3 described above) defined in the preamble of abovementioned EP 0765203B1 (also published as U.S. Pat. No. 5,832,807), in particular in scrap shears, have proved to be functional with regard to (cutting) impact damping; however, they are in need of improvement in the context of a complex hydraulic problem. With regard to the effectiveness of the technical means of the function described in step 4, this would only be partly realizable and only if additional means were to be used. This means at least a considerable cost outlay.
According to DE 4312283A1, the search for a solution approaches, such as, for example, in large, but continuously working channel baling presses, a control for a hydraulic heavy-duty actuator which is to be controlled under load from an extended working position into a return movement, a hydraulically pressurized working space of the actuator being connected to a return line of low pressure via a directional control valve, the valve position of which is set hydraulically via a control line which has at least one choke orifice.
In this case, a pressure-stabilizing delay element is connected in between the directional control valve and the choke orifice.
If the mode of operation of this control is analyzed with regard to the machine of the design mentioned at the beginning, valve operation for reducing relief impacts is certainly presented and a bracketed pressure from a system is reduced as smoothly as possible in order to reduce impacts extending into the tank line.
However, the rapid buildup of a counter-pressure in a cylinder and then also thus the limiting of this counter-pressure by this measure is not possible.
An approach to the solution of the present problem does not follow from this disclosure.
The aim of building up a counter-pressure as rapidly as possible and of limiting it in a cylinder of the machines of the generic type, taking into account their specific mode of operation, also cannot be achieved by the solution according to EP 1186783A1. According to this patent, only the oil flowing off from one cylinder is used for generally applying pressure to and driving a second cylinder. This effect is assumed to be known in order to actually fulfill the basic function of the hydraulic control in these machines.
It can therefore be stated that the problem of the (cutting) impact damping by means of hydraulic control for presses, and in particular shears for cutting metal scrap, defined at the beginning has hitherto not been solved comprehensively within the scope of the entire hydraulic system.
Thus, the volume of the hydraulic medium provided by the pumps in hydraulic controls for machine tools such as presses, and in particular shears for cutting metal scrap, constitutes a limit to the speed of the sequences of hydraulic cylinders, since the hydraulic medium flowing to the tank remains unused as a rule. There is also that fact that perpendicularly working hydraulic cylinders, such as in presses or shears, with attached masses of, for example, tools impose special demands on the hydraulic control. If the annular space of the cylinder is simply relieved toward the tank during the downward travel of this cylinder, the extension being effected downward, the piston of the cylinder could descend due to the dead mass and the attached load and could possibly run in advance of the action of the pumps. This has an adverse effect at least on the service life of the cylinder seals and may also be disadvantageous for the entire hydraulic system.
Finally, in conventional hydraulic controls for uses described above, with the simultaneous upward movement of two cylinders, the cycle time of the processing operation, such as after the shearing for example, is considerably restricted.